Cross-metathesis of vinylsilanes with olefins in the presence of Grubbs catalyst

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TETRAHEDRON LETTERS Tetrahedron Letters 42 (2001) 1175–1178

Pergamon

Cross-metathesis of vinylsilanes with olefins in the presence of Grubbs’ catalyst

Cezary Pietraszuk,â,b Helmut Fischer,^b Małgorzata Kujawaâ and Bogdan Marciniecâ,*

aFaculty of Chemistry,Adam Mickiewicz University,60-780 Poznan´,Poland

bFachbereich Chemie,Universita¨t Konstanz,Fach M 727, 78457Konstanz,Germany Received 18 September 2000; revised 2 November 2000; accepted 29 November 2000

Abstract—Effective cross-metathesis of H2CC(H)SiR3, where SiR3=Si(OMe)3, Si(OEt)3, Si(OSiMe3)3, with selected olefins in the presence of (PCy3)2Cl2Ru(CHPh) (I) is described. Treatment of p-substituted styrenes, 1-alkenes and selected allyl derivatives H2CCHCH2R% (R%=SiMe3, Si(OEt)3, Ph, OPh) with an excess of H2CC(H)SiR3 results in the formation of the respective cross-metathesis products with good yields and selectivities. The metallacarbene mechanism of the process is discussed. © 2001 Elsevier Science Ltd. All rights reserved.

Silyl olefins constitute an important class of compounds widely applied in organic synthesis.¹ Numerous methods for their preparation have been reported.^1b,c Cata- lytic methods include the hydrosilylation of alkynes, the dehydrogenative silylation of alkenes and the hydro- genation of alkynylsilanes.^1c The catalytic silylation of olefins by vinylsilanes developed in our group is an effective and general method for the preparation of alkenylsilanes.² The reaction, which resembles cross- metathesis, however, proceeds by a non-metallocarbene mechanism involving activation of CH and SiC bonds according to Scheme 1.³

Unfortunately, the formation of some amounts of isomerization products cannot be avoided when using this method.⁴

The development of well-defined, functional group tol-

erant ruthenium and molybdenum metathesis catalysts has opened new opportunities for applying metathesis in organic synthesis.⁵Important progress has also been made in developing efficient and highly selective cross- metathesis systems.⁶

Recently, we reported on the high catalytic activity of Grubbs’ catalyst in the cross-metathesis of vinylsilanes and vinylsiloxanes with styrene (Scheme 2).⁷ High yields and selectivities were obtained for vinyltrialkoxy- and vinyltrisiloxysilanes under very mild conditions (room temp.).

In this paper we describe new examples of effective cross-metathesis of vinyltrialkoxy- and vinyltrisiloxysilanes with aryl-, alkyl-, and allyl-substituted olefins catalyzed by complex I.⁸

The reactions proceeded according to Eq. (1):

Scheme 1.

Scheme 2.

* Corresponding author.

PII: S 0 0 4 0 - 4 0 3 9 ( 0 0 ) 0 2 2 0 1 - 2

First publ. in: Tetrahedron Letters 42 (2001), pp. 1175–1178

Konstanzer Online-Publikations-System (KOPS) URL: http://www.ub.uni-konstanz.de/kops/volltexte/2007/2387/

URN: http://nbn-resolving.de/urn:nbn:de:bsz:352-opus-23879

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C.Pietraszuk et al./Tetrahedron Letters42 (2001) 1175–1178 1176

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Table 1. Cross-metathesis of vinylsilanes with selected olefins

H2CCHSiR3 Olefin Conversion of olefin Yield of (isolated yield of) (E+Z) E/Z Yield of R%HCCHR%[%]

R%HCCHSiR3[%]

(vinylsilane)^a[%]

72 (100)â 68, 95â, (85)â

H₂CCHSi(OEt)₃ 4-Cl-Styrene E Traces

H₂CCHSi(OEt)₃ Styrene 70 (100)â 67, 95â, (88)â E Traces

H₂CCHSi(OEt)₃ 4-Me-Styrene 100^b 95^b E Traces

100^b 95^b

4-OMe-styrene E

H₂CCHSi(OEt)₃ Traces

1-Hexene

H₂CCHSi(OEt)₃ 100 75 9/1 10

80 60

H₂CCHSi(OMe)₃ 1-Hexene 10/1 8

90 72, (67)

1-Hexene 10/1

H₂CCHSi(OSiMe₃)₃ 7

1-Decene

H₂CCHSi(OEt)₃ 75 60 10/1 5

H₂CCHSi(OEt)₃ Allyl-SiMe₃ 100 95, (88) E Traces

75 71

Allyl-Si(OEt)₃ 15/1

H₂CCHSi(OEt)₃ 0

Allyl-Ph

H₂CCHSi(OEt)₃ 85 68 10/1 Traces

80 72 5/1

H₂CCHSi(OEt)₃ Allyl-OPh Traces

Reaction conditions: [(PCy₃)₂Cl₂Ru(CHPh)]:[H₂CCHSiR₃]:[olefin]=5×10⁻²:5:1; CH₂Cl₂, reflux, 3 h.

a[Ru]:[H2CCHSiR3]:[olefin]=5×10⁻²:1:3.

b1 h.

The results obtained are summarized in Table 1.⁹Prod- ucts were isolated and characterized spectroscopically.¹⁰ Since vinylsilanes were found to be inactive for the conversion to self-metathesis products,⁷the vinylsilanes could be used in excess. Thus, it was possible to mini- mize the role of competitive olefin self-metathesis.¹¹ Efficient reactions were observed for p-substituted styrenes, 1-alkenes, phenyl-, phenoxy-, trimethylsilyl-, and triethoxysilyl-substituted allyl compounds. In con- trast, no reaction was observed for allylamine,¹² allyl- methyl thioether¹³ and allyl chloride. In earlier studies these functional groups containing allyl derivatives were found to deactivate Grubbs’ type carbene complexes. Vinylsilanes containing one or more methyl substituents at silicon gave only traces or no cross- metathesis products. Similar results were obtained in the reactions with styrene.⁷ Removal of ethylene was found to be crucial for an increase of reaction efficiency.⁷ A high metathesis conversion was achieved by effective stirring and heating of the reaction mixture in CH₂Cl₂ to a gentle reflux. All reactions proceeded highly stereoselectively with a strong preference for the formation of theEisomer. The reactions of vinylsilanes with substituted styrenes⁷ as well as with the unsubsti- tuted styrene and with allyltrimethylsilane even afforded exclusively (within detection limits) the E isomer.

Based on the results of our earlier study,⁷a metallacarbene mechanism for these cross-metathesis reactions is postulated (Scheme 3).

The benzylidene complex I reacts with vinylsilane to form the methylidene complex II and silylstyrene.⁷

Compound IIthen reacts with the olefin H₂CC(H)R% to give the alkenylidene complexIII and ethylene. This reaction is a part of the commonly accepted metathesis mechanism.¹⁴The competitive reaction ofIIwith vinyltrialkoxy or vinyltrisiloxysilane leads to an exchange of methylidene units but does not produce the silylcarbene complex.⁷By reaction of vinylsilane with IIIthe cross-

Scheme 3. Proposed catalytic cycle for the cross-metathesis of vinylsilanes with olefins.

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metathesis product is formed. The relatively high catalyst concentration that has to be used is a consequence of the instability of the methylidene complex IIunder the conditions used.¹⁵ A detailed analysis of the reaction mixture indicates that, in addition to cross- metathesis and olefin metathesis products, also minor amounts of silylstyrenes—and for the reactions of vinyltrialkoxysilane—traces of tetraalkoxysilane (alkyl orthosilicate) are produced. The formation of silylstyrenes (Scheme 3) in the reaction of vinylsilanes withIhas been described earlier in more detail⁷and confirms the mechanistic scheme proposed above. Traces of Si(OR)₄ found in the reaction mixture are presumably formed by catalytic redistribution at silicon.¹⁶ Double-bond migration (olefin isomerization) was not observed in the starting olefins or in the cross-metathesis products. This confirms that, under the conditions used, hydride complexes are not generated in situ. Thus, cross-metathesis offers the advantage of avoiding olefin isomerization whereas in most other catalytic systems involving hydride catalysts isomerization is inevitable. The gener- ation of RuH complexes in systems containing initially catalyst I at temperatures ]60°C was reported very recently.¹⁷

In conclusion, the efficient and selective cross-metathesis of vinyltrialkoxy- and trisiloxysilanes with p-substi- tuted styrene-, 1-alkene-, and allyl-derivatives in the presence of (PCy₃)₂Cl₂Ru(CHPh) (I) offers an interest- ing route to unsaturated organosilicon compounds. In addition, the successful reactions with H₂CC(H)Si(OSiMe)3demonstrate the potential of this process for modification of poly(vinyl)siloxanes.

Acknowledgements

C.P. acknowledges a research grant from the Alexander von Humboldt Foundation. This work was also sup- ported by a grant no. T09A 139 19 from the State Committee for Scientific Research (Poland).

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9. General procedure for the catalytic cross-metathesis An oven dried flask equipped with a condenser and a magnetic stirring bar was charged under argon with CH2Cl2 (5 ml), decane or dodecane (internal standard), vinylsilane (vinylsiloxane) (3.14×10⁻³mol) and the respective olefin (6.28×10⁻⁴ mol). The reaction mixture was stirred and heated in a water bath to maintain a gentle reflux. Then ruthenium benzylidene complexI(3.14×10⁻⁵ mol) was added and the reaction was controlled by GC.

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Representative procedure for the synthesis of alkenylsilanes The reaction was carried out as described above with catalyst, reagents and solvent in ten times greater amounts. No standard was added. Reaction time: 3 h.

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10. Spectroscopic data of the selected new products:

E-(EtO)3SiCHC(H)CH2SiMe3:¹H NMR (CDCl3, ppm, coupling constants in Hz),d: 0.00 (s, 9 H, SiMe3); 1.21 (t, J=6.9, 9 H, 3×CH₃); 1.69 (dd,J=8.0, 1.4, 2 H, CH₂Si);

3.78 (q,J=6.9, 6 H, 3×SiOCH2); 5.19 (dt,J=18.7, 1.4, 1 H, CHSi), 6.41 (dt, J=18.7, 8.0 Hz, 1 H, CH). ¹³C NMR (CDCl3, ppm)d:−2.02 (SiMe3); 18.21 (CH3); 28.89 (CH₂Si); 58.27 (OCH₂); 116.75 (CHSi); 151.02 (CH).

MS,m/z(%): 73 (97), 79 (14), 119 (46), 133 (45), 135 (44), 143 (34), 158 (64), 159 (68), 163 (100), 187 (51), 207 (49), 232 (44), 233 (77), 261 (54), 276 (M⁺, 51); M⁺ found=

276.15656, calculated for C12H28O3Si2=276.15771.

E-(Me₃SiO)₃SiCHC(H)C4H₉: ¹H NMR (CDCl₃, ppm, coupling constants in Hz),d: 0.10 (s, 27 H, SiMe3); 0.90 (t,J=6.8, 3 H, CH₃); 1.26–1.44 (m, 4 H, 2×CH₂); 2.07–

2.14 (m, 2 H, CH2); 5.36 (dt,J=18.4, 1.6, 1 H,CHSi);

6.20 (dt, J=18.4, 6.3, 1 H, CH). ¹³C NMR (CDCl₃, ppm) d: 1.74 (SiMe3); 13.92 (CH3); 22.21 (CH2); 30.62 (CH2); 36.05 (CH2); 123.79 (CHSi); 150.24 (CH). MS, m/z(%): 59 (8), 73 (100), 191 (8), 193 (24), 207 (45), 295 (14), 363 (23), 378 (M⁺, 1).

E-(EtO)₃SiCHC(H)C6H₄Cl: ¹H NMR (CDCl₃, ppm, coupling constants in Hz),d: 1.26 (t,J=6.8, 9 H, CH3);

3.88 (q, J=6.8, 6 H, CH₂O); 6.14 (d, J=19.2, 1 H,

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CHSi); 7.35 (d, J=19.2, 1 H,CH); 7.31–7.39 (m, 4H, C6H4Cl). ¹³C NMR (CDCl3, ppm) d: 18.2 (CH3); 58.6 (CH₂O); 118.6 (CHSi); 127.9, 128.7, 134.4, 136.0 (C₆H₄Cl); 147.6 (CH). MS, m/z (%): 79 (16), 91 (8), 107 (10), 115 (11), 119 (34), 135 (11), 163 (100), 164 (11), 179 (2), 181 (1), 195 (1), 225 (2), 255 (0.3), 300 (M⁺, 0.2); M⁺ found=300.09478, calculated for C₁₄H₂₁ClO₃Si=300.09485

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